PALAEOPRESSURES OF SOUTH INDIAN 2-PYROXENE GARNET GRANULITES FROM THERMOCHEMICALLY CALIBRATED CMAS BAROMETERS

The enthalpy of reaction of plagioclase and pyroxene to produce garnet and quartz has been a major source of error in granulite geobarometry because of relatively uncertain enthalpy values available from high-t emperature solution calorimetry and compiled indirectly from experimen tal phase equilibria. Recent, improved calorimetric measurements of DE LTAH(R) are shown to yield palaeopressures which are internally consis tent between orthopyroxene and clinopyroxene calibrations for many Sou th Indian granulites from the Archaean high-grade terranes of southern Karnataka and northern Tamil Nadu. This represents a considerable imp rovement over previous calibrations, which gave disparate results for the two independent barometers involving orthopyroxene and clinopyroxe ne, requiring a 2-kbar 'empirical adjustment' to force agreement. Pala eopressures thus calculated for 30 well-documented two-pyroxene garnet granulites from South India give internally consistent pressures with a mean of 8.1+/-1.1kbar at 750 degrees C, consistent with the presenc e of both kyanite and sillimanite in many areas. Those samples for whi ch garnet-pyroxene exchange thermometers give plausible granulite-rang e temperatures and whose minerals are minimally zoned give the best ag reement of the two barometers. Samples which yield low palaeotemperatu res and different rim and core compositions of minerals yield pressure s for the orthopyroxene assemblage as much as 2kbar lower than for the assemblage with clinopyroxene. This disparity probably represents pos t-metamorphic-peak re-equilibration. We conclude that considerable con fidence may be placed in geobarometry of two-pyroxene granulites where apparent palaeotemperatures are in the granulite facies range (>700 d egrees C) and where mineral zonation is minimal. Of the several possib le sets of activity-composition relations in use, those constructed fr om analysis of phase equilibria give slightly higher palaeopressures a nd appear more consistent with analytical data from the Nilgiri Hills uplift, where kyanite is the only aluminium silicate reported to be st able in peak-metamorphic assemblages. The present results support a pa laeopressure gradient, increasing generally from south to north. acros s the Nilgiri Hills as inferred by previous geobarometry.